The invention relates to a picture display device comprising an evacuated envelope, a first side of which is provided with an electroluminescent display screen, an opposite side is provided with an electron gun and in which deflection means are arranged between the electron gun and the display screen with which means, at least during operation, an electron beam generated by the electron gun can be deflected, the electron gun having a portion generating at least an electron beam and being provided with a main lens system having a first electrode, a final electrode and at least one intermediate electrode across which a main lens voltage is gradually applied step-wise during operation so as to form an electron-optical main focusing lens. The invention also relates to an electron gun for use in such a device.
Such a device is known from European Patent Specification 302 657. The electron gun described in this Specification comprises two intermediate electrodes between the first electrode and the final electrode of the main lens system and, in comparison with other, more conventional electron guns, it comprises a relatively large number of electrodes For this reason such a main lens is commonly referred to as DML (Distributed Main Lens), MSFL (Multi-Stage Focus Lens) or MEL (Multi-Element Lens). The separate electrodes of the main lens system in the known device are interconnected by means of a resistive voltage divider so that the main lens voltage is gradually distributed step-wise across the electrodes during operation in order to reduce the magnitude of potential jumps in the main lens system. This leads to considerably improved lens properties as compared with more conventional guns in which the main lens voltage is entirely applied across only two electrodes. Notably spherical aberrations can be adequately suppressed to relatively large electron beam currents without an increase of the lens diameter being required.
To enhance the lens action, the intermediate electrodes of the main lens system of the electron gun of the known device are made of a relatively thick sheet material. A thickness of minimally approximately 1-2 mm appears to be desirable in this respect. A drawback of the use of such a thick sheet material is, however, that (mechanical) steps of processing this material are impeded. For example, in such a thick sheet material it is difficult to punch, for example the apparatus for passing the electron beam(s) in the electrodes and consequently considerably longer and more costly techniques such as, for example laser cutting will have to be used. Moreover, the risk of deviations and errors in the apertures and the edges of the electrodes is larger as the sheet material is thicker. These drawbacks are all the more troublesome if the electron gun is to be mass-produced.